Hmm…works fine on all machines that i’ve tried (using 1.5.x and 1.4.2). Could it be a problem with Java 6? The last version that i tried couldn’t webstart anything correctly, but that was some month ago (i think…).
I’m using Java 6 too and it doesn’t work. Could it be that you need to specify sections in the JNLP for v6? Or have the version strings wrong?
Cas 
I don’t know what was wrong…i’ve installed Java6 beta myself and after some tries (where is worked fine), it stopped working with the same exception. I’ve rewritten the jnlp and now it works fine for me even when using Java6. Can somebody please verify this!?
Edit: On my Java6beta, i’ve to start some webstart apps twice, because after downloading, it gives me a class cast exception inside JWS, when it tries to cast a File into a CacheJarFile. And sometimes, i’m getting Exceptions in the InputStream while downloading…could it be that this beta is quite…well…beta?
Works for me now!
Really fun to play now! I like the new models. However I did find one thing I’d like to see. After capturing a new model, I wasn’t really sure if I liked it as much. I couldn’t remember what the green/red/yellow bars stood for and no one ever stands still long enough for you to see the icons, so I hit Escape hoping to find an info screen. Options didn’t have it, so I hit Main Menu hoping for more options. Not only did I not find any more information, but my game ended and I couldn’t resume! 
I’m currently working on the consoles the classic game had. By using them (i.e. bumping into them while in transfer mode) you can access a level map and get some information about the droids. When not in use, they’ll show random pictures…to be honest, that’s all they do in their current state… 
Currently, it’s looking like this:
Another 200 years and it’ll be finished 8)
Cas
I think that it is finished…and never will…